Part Five - Circle Of Confusion

Lens Knowledge

With all the new technology we have around us today it's often the fundamentals and basics of understanding that can often be left out and forgotten. For newcomers to the business there is often much jargon to understand, so I wanted to cover some of the understanding of the technology and common areas of knowledge. I thought I would start with lenses and how they work to create an image. One area as a technical area covering the definition of Circle of Confusion, and the other is perhaps less commonly discussed, that of Bokeh, and how a lens represents points of ‘out of focus’ light.

First I will start with the Circle of Confusion, keep on reading for the Bokeh article

Circle of Confusion

In cinematography we often talk about circles of confusion (CoC) as a measure of how sharp an image is defined with a lens. This is an important technical method of accurately describing how sharp a lens is and what depth of field the lens has at a particular size format. It is however a little more complicated than that, and the definition of a lens relies on many areas of assumptions.

A circle of confusion is the size of the optical spot of light when light rays are focussed by a lens and a cone of light rays converge at a certain point, such as an image sensor or film plane. The cone of light rays form a point of light a certain size, and the degree to how big the point is defines the circle of confusion. Put simply, a larger circle of light means it is less focussed and a smaller point of light is a more accurately focussed circle. The size of the circle can be measured, and the value used to define what is accurately in focus and what is not.

The famous original 35mm Kelly calculator from the GBCT was in its day the de facto tool every focus puller had in their back pocket, and it was referred to on every shot to enable the focus puller to check the depth of focus each shot had available. The Kelly defined what is acceptable in focus when a certain focal length lens was set at a particular aperture and focussed on a certain distance. The scale is a simple to use circular sliding scale, like a circular slide ruler. The Kelly is still being used, but the wide variety of sensor sizes on cameras made it inevitable that many digital apps were developed to take account of the many permutations.

The values of CoC are most often measured with each image format, as the acceptable CoC on a projected image is relative to the size of the image format (for example 16mm and 35mm full frame, or Super 35 ). No lens will focus all rays perfectly, so the area of the focussed spot of light is defined as a spot rather than a point of light. The smallest spot a lens is able to produce is often referred to as the circle of least confusion.

How a viewer defines how much of an image is in focus is also relative to viewing distance, the size of the screen and magnification of the image, the actual visual acuity of the viewer, viewing conditions, the action in the shot and the contrast of the image, along with other factors. If you were to stand right next to a movie theatre screen it would be very obvious the picture is only sharp when sitting at the optimum viewing distance away.

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